1. Field of the Invention
The present invention relates to a method of purifying isophthalonitrile including separating isophthalonitrile from a gas produced through reaction of m-xylene with ammonia and an oxygen-containing gas in the presence of a catalyst. Isophthalonitrile is useful as a raw material for producing synthetic resins and agrochemicals and an intermediate material for producing compounds such as amines and isocyanates.
2. Background
A method of reacting, in the presence of a catalyst, an organic-substituent-containing carbon-ring or heterocyclic compound with ammonia and an oxygen-containing gas is called ammoxidation, and is generally employed for producing nitrile compounds through a vapor-solid fluidized catalytic process.
A variety of methods for separating a nitrile compound from a gas produced through ammoxidation have already been known. For example, Chemical Engineering (November 1971, pp. 53-55) discloses a method for depositing isophthalonitrile, in which a gas produced through ammoxidation of m-xylene so as to produce isophthalonitrile is introduced into a scrubber and is cooled with water, then the obtained slurry of isophthalonitrile is introduced into a filter, to thereby isolate crystals of isophthalonitrile, and the crystals are dehydrated and dried, to thereby yield a final product.
Process Handbook (published in 1976, edited by The Japan Petroleum Institute) discloses the MGC-Badger isophthalonitrile process, in which isophthalonitrile contained in a gas produced through reaction is trapped by an organic solvent; the isophthalonitrile-trapped liquid is transferred to a solvent recovery column for the removal of solvent from the column top and crude isophthalonitrile is recovered from the bottom; and the crude isophthalonitrile is supplied to a rectification column, whereby purified isophthalonitrile is recovered from the column top.
In the method described in Chemical Engineering in which a gas produced through ammoxidation of m-xylene so as to produce isophthalonitrile is introduced into a scrubber and is cooled with water, by-products generated during ammoxidation are also deposited with isophthalonitrile. Thus, an additional purification step is required in order to obtain isophthalonitrile of high purity.
The method described in Process Handbook employing trapping by an organic solvent enables obtaining of high-purity isophthalonitrile. However, the method poses the following problems among others. (1) When a sublimable high-melting-point substance such as isophthalonitrile is separated through distillation under reduced pressure and removed from the top of the distillation column, isophthalonitrile may be solidified due to overcooling, since the condensation temperature and the melting point are close to each other in a high vacuum. (2) Due to high-temperature operation, vapor pressure of isophthalonitrile becomes high, and isophthalonitrile migrates to a vacuum evacuation system, to thereby deposit crystals thereof and cause plugging. (3) To prevent this, there must be taken measures including provision of a scrubber between the condensation section and the vacuum evacuation system. (4) Generally, in the presence of impurities such as high-boiling-point by-products generated during ammoxidation, ammoxidation catalyst, and metallic components, isophthalonitrile is unstable to heat and readily undergoes undesirable change or deterioration. Thus, when isophthalonitrile is exposed to high temperature during distillation, significant portions of isophthalonitrile are lost.
Thus, an object of the present invention is to provide a method for producing high-purity isophthalonitrile at high yield constantly for a long period of time in an industrially advantageous manner while, during purification through distillation of isophthalonitrile produced by ammoxidation, loss of isophthalonitrile is minimized and plugging due to deposition of crystals of isophthalonitrile from a condensation to a vacuum evacuation system is prevented.
In an attempt to solve the aforementioned problems, the present inventors have carried out extensive studies focusing on the methodology for producing isophthalonitrile, and have found that by trapping isophthalonitrile contained in a gas produced through ammoxidation by an organic solvent having a boiling point lower than that of isophthalonitrile; removing high-boiling-point impurities in a first distillation step; and in a second distillation step, separating the organic solvent and recovering isophthalonitrile from the bottom of the column, deterioration of isophthalonitrile can be suppressed, and provision of a special apparatus for preventing deposition of isophthalonitrile crystals in a vacuum evacuation system can be eliminated. The present invention has been accomplished on the basis of this finding.
Accordingly, the present invention provides a method for purifying isophthalonitrile including separating isophthalonitrile from a gas produced by causing m-xylene to react with ammonia and oxygen-containing gas in the presence of a catalyst, which method comprises the following steps:
a) a trapping step for bringing the gas into contact with an organic solvent having a boiling point lower than that of isophthalonitrile, to thereby trap isophthalonitrile in the organic solvent;
b) a high-boiling-point impurity separation step for distilling a liquid in which isophthalonitrile is trapped in the trapping step, to thereby recover isophthalonitrile and the organic solvent from the top of the column and separate at the bottom of the column impurities having boiling points higher than that of isophthalonitrile; and
c) a rectification step for subjecting isophthalonitrile and the organic solvent resulting from the high-boiling-point impurity separation step to rectification, to thereby recover the organic solvent from the top of the column and recover liquefied isophthalonitrile of high purity at the bottom of the column.